1. Field of the Invention
The present invention relates generally to meters and more specifically to a meter for making accurate and precise measurements of the transmissivity of floppy disks to radiation in the infrared spectrum.
2. Description of the Prior Art
An important vehicle for information storage is the floppy disk also referred to as a flexible disk or diskette. As the name implies, a floppy disk is a thin round piece of flexible magnetic recording material, such as heavy oxide coated mylar based magnetic tape recording material. The floppy disk contains a large centrally located spindle hole and a small index hole, a common location for which is near the spindle hole. Additionally, some floppy disks, called hard sectored disks, each contain a plurality of small sector holes located at equal angular increments around the disk at the same radius as the sector hole. Further, floppy disks are manufactured in a variety of sizes including a size which resembles a 45 RPM record, referred to as 8 inch, and a smaller size, referred to as 51/4 inch or simply 5 inch.
Many floppy disks are enclosed, each between a pair of soft, low friction, anti-static liners within a square shaped protective envelope, also commonly referred to as a cartridge or jacket. Each such envelope contains a number of openings which are aligned on opposite sides of the envelope and open through the liners to provide access to the floppy disk. A pair of large round centrally located openings permit the edge of the floppy disk around the spindle hole to be engaged between a hub and a motor driven spindle to rotate the disk within its envelope. A pair of radially aligned slotted openings permit a pair of opposed read/write heads, or a head and a pressure pad, to access opposite surfaces of the floppy disk along a radial line over the extent of the information storage area. Further, a pair of small openings are located at the same radius as the index and sector holes.
The area of the floppy disk upon which information is recorded is divided into a plurality of imaginary concentric rings called tracks. Each of the tracks is divided into a plurality of equal angular portions called sectors. Sector synchronization is provided by the index hole. On floppy disks which lack sector holes, called soft sectored disks, sector delineation and identification is provided by information recorded directly on each of the tracks. Sector holes, on hard sectored disks, provide this function. Each sector hole delineates a sector which is identified by counting the number of sector holes from the index hole.
Detection of the index hole and, on hard sectored floppy disks, the sector holes is provided by a sensor that includes the combination of a light emitting diode (LED) and a photodetector. The LED is disposed on one side of the floppy disk aligned with the index hole opening in the disk envelope so as to illuminate the disk therethrough. The photodetector is disposed on the opposite side of the floppy disk optically aligned with the LED so as to detect illumination passing through the index hole or each of the sector holes when one is rotated into alignment with the optical path.
Unfortunately, the index hole sensor is highly susceptible to false triggering caused by LED generated illumination being transmitted through the floppy disk. To minimize such problems standards are being developed for the maximum acceptable floppy disk transmissivity, i.e. the ratio of the energy transmitted through the disk to that which is incident thereon.
A proposed standard promulgated by the American National Standards Institute in a publication known as "The Twelfth Draft of the American National Standard for Single Sided Unformatted Flexible Disk Cartridges" establishes a maximum floppy disk transmissivity to infrared radiation of 900 nanometers wave length of 1/2% where the maximum transmissivity is defined by a reading obtained with a neutral density filter of known transmissivity between 0.45% and 0.55% calibrated with 900 nanometer radiation. Obviously, both the accuracy and the precision with which the transmissivity of a floppy disk may be measured is of considerable moment. It is important to insure that floppy disks meet this proposed standard without rejecting good disks or imposing unnecessarily stringent manufacturing requirements.
Relevant to the problems of measuring transmissivity are a number of prior art disclosures. Henry P. Kalmus et al in U.S. Pat. No. 2,500,547 review the use of a rotating shutter to modulate a light source used in transmissivity or reflectivity measurements to permit the detected transmitted or reflected energy to be amplified by an AC amplifier rather than a relatively unstable DC tube amplifier. Further, Kalmus et al disclose a densitometer which includes a multi-vibrator or thyratron oscillator used to modulate a lamp to a depth of approximately 100% and a tuned amplifier to amplify the output of a photo-sensitive element to develop a meter driving signal.
A densitometer for chemical analysis is disclosed in U.S. Pat. No. 3,807,875 by David J. Fischer et al. The disclosed densitometer includes a mono-stable multivibrator for developing pulses which are used to drive a gallium arsenide LED to develop radiation at a wave length of approximately 0.9 microns for illuminating materials to be measured. Also included are a semiconductor phototransistor for detecting LED radiation attenuated by the sample, an amplifier for amplifying the detected signal, and a peak detector and hold circuit gated responsive to each of the pulses of the mono-stable multivibrator to generate a meter driving signal that is proportional to the peak of the amplified detected signal. Although indicating that noise is a problem in making high absorption measurements, Fischer et al indicate that they avoid the problem by only making relative, rather than absolute, measurements.
To provide an alternative to the use of a flashlight in detecting hydrocephalus, Curtis C. Johnson discloses in U.S. Pat. No. 3,674,008 an apparatus for measuring the optical density of a human skull. The disclosed apparatus includes a triggering pulse generating oscillator and a strobe or flashlamp controlled thereby for illuminating a portion of a skull. Resultant illumination at another portion of the skull is detected by a photo-multiplier, filtered and stored in a sample-and-hold circuit responsive to the oscillator, to develop a display driving signal. It is suggested that the strobe and photo-multiplier may be replaced by a gallium arsenide diode and a silicon photo-detecting diode, respectively.
A meter for measuring the opacity of smoke discharged in the exhaust of a motor vehicle is disclosed by Richard Krukowski in U.S. Pat. No. 3,711,210. The disclosed meter employs a pulse generator driven gallium phosphide device for developing light pulses. A combination of a silicon phototransistor, a band pass filter tuned to the frequency of the light pulses, an amplifier, a detector and a hold circuit are employed to develop a meter driving signal that is proportional to the opacity of smoke between the gallium phosphide device and the photo-transistor. A tungsten biasing lamp and associated circuitry are also employed to maintain a constant voltage offset at the output of a photo-transistor to compensate for variations in ambient light.
Smoke detectors employing multi-vibrator driven light sources are disclosed in the U.S. Pat. No. 3,524,707 issued to Julian E. Hansen et al and U.S. Pat. No. 3,846,772 issued to William T. Peberdy. The Hansen disclosure employs a band pass filter in the light receiver circuitry. The Peberdy disclosure employs a gallium arsenide diode, emitting infrared radiation and an amplifier in the radiation detecting circuitry which is tuned to those frequencies that are produced by hot gases.
Also of interest is the apparatus for measuring drops or globules of liquid, such as oil, dispersed in another liquid, such as water, disclosed by Norman A. Lyshkow in U.S. Pat. No. 3,864,044 and the nephelometer disclosed by William B. Underwood in U.S. Pat. No. 4,118,625.
Although the above-mentioned references are of interest, the need to make accurate and precise measurements of the transmissivity of a floppy disk at an infrared wave length to insure compliance with disk standards presents unique problems not addressed by these references.